The present invention relates generally to intensity modulated radiation therapy. More particularly, the present invention relates to a method for verification of monitor units and fluence map in intensity modulated radiation therapy.
Intensity modulated radiation therapy (IMRT) is a new modality of radiation therapy that shows promise for significantly improving dose conformation to the target and dose avoidance to the sensitive structures. IMRT has emerged from the developments of inverse planning and computer-controlled delivery using collimators (MiMiC) or multileaf collimators (MLC). An important problem in IMRT planning verification is how to efficiently verify the monitor unit (MU) calculation of an inverse planning system. An intensity modulated beam consists of a set of beamlets and is designed on a patient-specific basis. The beam can literally have any fluence profile and it is no longer possible to follow the manual MU check procedure of conventional conformal radiation therapy to validate an IMRT plan. Currently, the MU verification of IMRT is labor intensive, time consuming, costly and an institution dependent method.
One way to verify the MU or dose calculation of an IMRT treatment planning system is to sum the fractional MUs corresponding to the segmented fields. The MU calculation for a multileaf collimated static field has been described in the literature (Georg D and Dutreix A 1997. A formalism to calculate the output ratio in a mini-phantom for a GE multileaf collimator, Phys. Med. Biol. 42, 521-536). Boyer et al. (1999) have illustrated how MU settings are derived from the prescribed dose in CORVUS inverse planning system (NOMOS Corporation, Sewickley, Pa.) and experimentally verified the system using ionization chamber in a water phantom (Boyer A L, Xing L, Ma C, Curran B, Hill R, Kinia A and Bleier A 1999. Theoretical considerations of monitor unit calculations for intensity modulated beam treatment planning. Med. Phys. 26, 187-195). However, they did not touch the issue of how to use a computer to independently verify the MU settings provided by an inverse planning system. Geis and Boyer (Geis P and Boyer A L, Use of a multileaf collimator as a dynamic missing-tissue compensator, Med. Phys. 23, 1199-1205, 1996) investigated the feasibility of replacing conventional physical missing-tissue compensators by using dynamic multileaf collimators. Geis and Boyer (1996) introduced a method to calculate MU for dynamic compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculation. The formula ignored the MLC leaf transmission and the MLC movements were designed to mimic a physical compensator. To obtain the MU of a dynamic compensator, it requires to know the MU setting of the corresponding physical compensator, which is generally not available and rendered the approach invalid for MU verification of an intensity modulated field. Kung and Chen (1999) have applied the Clarkson method to directly calculate the dose of an intensity modulated field at central axis and then compared the result with the result from the treatment planning system. (Kung J and Chen G 1999. A modified Clarkson integration (MCI) for IMRT. Med. Phys. 26: 1135; Kung J, Chen G Kuchnir F 2000, A monitor unit verification calculation in intensity modulated radiotherapy as a dosimetry quality assurance, Med. Phys. 27, 2226-2230). Their method has two major deficiencies. First, it did not separate the dosimetric effect of the dynamic modulation from the beamlet kernels and was thus applicable only when the Clarkson method was used for dose evaluation. It is practically impossible to generalize their approach for dose calculation based on other more advanced methods. The validity of the Clarkson-based approach becomes questionable as the spatial resolution of intensity modulation increases. The approach is also problematic for accelerators with variable jaw settings during IMRT delivery. Second, their approach fails to yield useful information when the verification point is located in a low dose region.
A key issue of quality assurance in IMRT is to establish a set of empirical criterions that are clinically acceptable and technically achievable. As far as the point dose is concerned, AAPM TG-40 recommends checking the point dose near the center of the tumor and a disparity should be resolved before commencing or continuing treatment if the difference is more than 5% [See for instance, G J Kutcher, et al., 1994 Comprehensive QA for radiation oncology: report of AAPM Radiation Therapy Committee Task Group 40. Medical Physics, 21(4): p. 581-618.]. Experience by the present inventor with IMRT dose validation in a high dose region indicates that dose agreement within 3%xcx9c5% is adequate and reflects the current standard practice. However, implementation of the criterion for all IMRT cases is complicated by the fact that the beam intensity is modulated and frequently the point of interest (POI) is located in the low dose region of one or more treatment fields. Generally, the uncertainty of dose calculation using simple techniques is higher in a low dose region due to the limited capability to model the MLC-modulated fluence and photon transport process in this situation. The relative dosimetric error can be as high as 5% to 40% in a low dose region when the data is normalized to the dose at the POI. In reality, a large relative error may rise from a true dosimetric error or simply because the point is in a low dose region which enhances the relative error.
Accordingly, there is a strong need to develop a general method for an independent MU or dose calculation of an intensity modulated photon field. Furthermore, there is a strong need to develop a method to verify high and low dose regions to provide better and unified quality assurance.
The present invention provides a general computer-implemented method and computer program for an independent MU or dose calculation of an intensity modulated photon field. The present invention provides a closed formula for point dose calculation in IMRT. Furthermore, the present invention provides a method to verify high or low dose regions in a treatment field. The dose at an arbitrary spatial point (either on the central axis or off-axis) is expressed as a summation of the contributions from all the beamlets, each is modulated by a dynamic modulation factor. Besides providing a clear physical picture, it allows one to implement the MU at different level of sophistication to meet the specific requirement of different system. Furthermore, the present invention provides a computer implemented method and computer program for the validation of MU setting when the verification point is in a low dose region using an inverted field approach. Finally, the present invention provides a computer implemented method and computer program for verifying a fluence map in a treatment field for intensity modulated radiation therapy.
It is the objective of the present invention to provide a calculation formalism for independent verification of the monitor units or point dose in either high or low dose regions in IMRT.
It is another objective of the present invention to provide a method of deriving the fluence map from the dynamic modulation factor distribution for independent fluence map calculation.
It is yet another objective of the present invention to verify or check both point dose and fluence map for the validation of an IMRT treatment plan.
The advantage of the present invention that is generally applicable and useful irrespective of the type of leaf sequence algorithm and delivery machines. The method of the present invention could also be applied for the MU verification of IMRT based on multivane intensity modulation collimators (MiMiCTM) and other types of delivery systems (e.g. IMAT devices). The present invention is therefore institution independent. It provides an automated computer-implemented method or program that generalizes and simplifies dose verification in IMRT.